Since a semiconductor light-emitting element has a small size and a long lifetime, existing light-emitting devices are in the process of being replaced by semiconductor light-emitting elements. For example, semiconductor laser diodes (LDs), which use a stimulated emission, are being used in display devices, storage devices such as optical disk devices, and the like. Semiconductor light-emitting diodes (LEDs), which use a spontaneous emission, are being used in display devices and the like because of increased brightness.
Recently, LEDs have been used for lighting. Since the LEDs are more efficient and generate less heat than incandescent lamps, it is considered that replacement of the incandescent lamps by the LEDs will continue to progress in the future. A LED potentially has a capability to increase the efficiency of converting electricity to light close to 100%. However, the LED has a problem for a decrease of an operating voltage. Moreover, in the replacement of the fluorescent lamps, there is a problem in efficiency, heat generation, and operating power source.
A nitride semiconductor is a strong crystal and therefore can work with a large current and a high voltage. For a semiconductor device other than a LED also, the nitride semiconductor may provide characteristics which exceed the performance of a semiconductor, such as gallium arsenide (GaAs), and silicon (Si). However, in a nitride semiconductor, it is difficult to provide an electrode having a low contact resistance, particularly to a p-type semiconductor layer.
With respect to nitride semiconductor light-emitting elements, there is a proposal to use a metal at least containing palladium (Pd) (refer to Japanese Patent No. 3233258) in order to provide an electrode to a p-type layer (hereinafter referred to as p-side electrode) with a low contact resistance. Additionally, technology has been disclosed where an oxide of nickel (Ni) and a palladium (Pd) metal are used (refer to Japanese Patent No. 3230463). However, in each of the above technologies, an operating voltage of a nitride semiconductor light-emitting element is high, and contact resistance is not sufficiently reduced.
If a contact resistance of an electrode to supply power to a semiconductor layer is high, a high operation voltage occurs and causes a problem for improving operating efficiency of a semiconductor device. Additionally, by heat generation at an interface between the semiconductor layer and the electrode, due to high contact resistance, the electrode is damaged as is the semiconductor layer in the vicinity of the electrode, which leads to degradation in reliability of the semiconductor device. Furthermore, a pad electrode, such as gold (Au), is provided on the electrode of the semiconductor device for wiring, such as wire bonding. Au can be diffused into the semiconductor layer through grain boundaries of the electrode metal. The diffused Au causes leakage of the semiconductor device and creates a reliability problem for the semiconductor device.